Abstract
Abstract. The source of bromine that drives polar boundary layer ozone depletion events (ODEs) is still open to some debate. While ODEs are generally noted to form under conditions of a shallow stable boundary layer, observations of depleted air under high wind conditions are taken as being transport-related. Here we report observations from Antarctica in which an unusually large cloud of BrO formed over the Weddell Sea. The enhanced BrO was observed over Halley station in coastal Antarctica, providing an opportunity to probe the conditions within an active "bromine explosion" event. On this occasion, enhanced BrO and depleted boundary layer ozone coincided with high wind speeds and saline blowing snow. We derive a simple model to consider the environmental conditions that favour ODEs and find two maxima, one at low wind/stable boundary layer and one at high wind speeds with blowing snow. Modelling calculations aiming to reproduce the wider regional or global impacts of ODEs, either via radiative effects or as a halogen source, will also need to account for high wind speed mechanisms.
Highlights
Ozone depletion events (ODEs) are well-documented phenomena that are observed in coastal areas of both the northern and southern polar regions during spring (e.g. Simpson et al, 2007 and refs therein)
Such a condition can be achieved under low wind speeds when the boundary layer is shallow and stratified, and as referred to above, this matches the conditions under which many ozone depletion events (ODEs) are observed
On 9 October 2007, satellite observations show that vertical column densities of BrO were enhanced across the Weddell Sea region of Antarctica
Summary
Ozone depletion events (ODEs) are well-documented phenomena that are observed in coastal areas of both the northern and southern polar regions during spring (e.g. Simpson et al, 2007 and refs therein). For a condensed phase that is restricted to the surface of the Earth, such as the majority of those proposed for the bromine explosion, the likelihood of reaction can be enhanced by increasing the concentration of reactants in air just above the Earth’s surface. Such a condition can be achieved under low wind speeds when the boundary layer is shallow and stratified, and as referred to above, this matches the conditions under which many ODEs are observed. The results challenge the conventional view that a stable shallow boundary layer is an essential pre-requisite for severe tropospheric ozone depletion and show that ODEs can propagate during extreme wind events
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